BASE64位转码有两种：
⼀种是图⽚转为Base64编码，这种编码是直接可以在页⾯通过<img src='base64编码'/>的⽅式显⽰
Base64 在CSS中的使⽤
.demoImg{ background-image: url("data:image/jpg;base64,/"); }
Base64 在HTML中的使⽤
<img width="40" height="30" src="data:image/jpg;base64,/" />
第⼆种中Base64编码转为字符流，这样使⽤要⿇烦⼀些，需要进⾏字符流解码为图⽚再访问。(后来发现这种是不含编码头的字符码，加上Base64位编码头(如data:image/png;base64,)也可以⽤⽅法⼀来直接显⽰。--update on 2015-3-25) 1.为什么要⽤到BASE64编码的图⽚信息
Base64是⽹络上最常见的⽤于传输8Bit字节代码的编码⽅式之⼀。Base64 主要不是加密，它主要的⽤ 途是把⼀些⼆进制数转成普通字符⽤于⽹络传输。由于⼀些⼆进制字符在传输协议中属于控制字符，不能直接传送需要转换⼀下。最常见的⽤途是作为电⼦邮件或WebService附件的传输编码. 2.base64编码定义
⽬前的internet e-mail标准--简单邮件传递协议(smtp)在rfc821中规定了两条重要但不难实现的限制：
1)邮件的内容必须全部为7-⽐特的美国ascii码。
2)每⼀⾏的长度不能超过1000的字符。
因此为了通过smtp⽤e-mail进⾏传送，内存的序列化对象必须转化为和以上相容的格式。
rfc1521提供了⼀个可⾏的⽅案。它定义了邮件的内容部分，使之能包涵多种形式的数据。这种标准就是⽬前众所周知的mime。 按照rfc1521编码过程为：输⼊是24个⽐特，输出是4个字节。24个⽐特输⼊组从左⾄右由3个8⽐特的输⼊组形成。这24个⽐特被看成4个连续的6⽐特组,⽽每个6⽐特输⼊组被翻译为base64码表中的⼀个数字。依次反复不断进⾏，直到全部输⼊数据转换完成。
如果最后剩下两个输⼊数据，在编码结果后加1个“=”；如果最后剩下⼀个输⼊数据，编码结果后加2个“=”；如果没有剩下任何数据，就什么都不要加，这样才可以保证资料还原的正确性。
完整的base64定义可见 RFC1421和 RFC2045。编码后的数据⽐原始数据略长，为原来的4/3。在电⼦邮件中，根据RFC822规定，每76个字符，还需要加上⼀个回车换⾏。可以估算编码后数据长度⼤约为原长的135.1%。
3.如何进⾏base64编码
Base64 使⽤US-ASCII⼦集的65个字符, 每个字符⽤6位表⽰
对于⽂本串，编码过程如下。例如"men":
先转成US-ASCII值.
"m"⼗进制 109
"e"⼗进制 101
"n"⼗进制 110
⼆进制 :
m 01101101
e 01100101
n 01101110
三个8位连起来是24位
011011010110010101101110
然后分成4个6位
011011 010110 010101 101110
现在得到4个值，⼗进制为
27 22 21 46
对应的 Base64 字符是 : b W V u
编码总是基于3个字符，从⽽产⽣4个Base64字符。
如果只是2个字符的数据，使⽤特殊字符"="补齐Base64的4字。
如，编码"me"
01101101 01100101
0110110101100101
011011 010110 0101
111111 (与，补⾜6位)
011011 010110 010100
b W U
b W U = ("=" 补⾜4字符)
于是 "bWU=" 就是"me"的Base64值.
如果只是2个字符的数据，如编码 "m"
01101101
011011 01
111111
011011 010000
b Q = =
于是 "bQ==" 就是"m"的Base64值.
4.显⽰被存储为Base64编码字符串的图⽚的例⼦
1)使⽤data: URI直接在⽹页中嵌⼊.
data: URI定义于IETF标准的RFC 2397
data: URI的基本使⽤格式如下：
data:[<MIME-type>][;base64|charset=some_charset],<data>
mime-type是嵌⼊数据的mime类型，⽐如png图⽚就是image/png。
如果后⾯跟base64，说明后⾯的data是采⽤base64⽅式进⾏编码的
<html>
<body>
<img
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</body>
</html>
这种⽅式,Firfox、Opera、Safari和Konqueror这些浏览器都已经⽀持，但是IE直到7.0版本都还没有⽀持.所以,⽐较好的做法是在服务器端将base64编码的字符串转换成byte流.这⾥，我提供了java的实现⽅法.
2)字符串转换成byte流，然后显⽰出来
在java中，提供了⼀个⽤于编码和解码(encode/decode )base64字符串和数据流的Java开源类库Java Base64.
服务器端代码
package test;
import java.io.*;
*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;
import sun.misc.BASE64Decoder;
import sun.misc.BASE64Encoder;
public class ImgShow extends HttpServlet {
public void doGet(HttpServletRequest req, HttpServletResponse res)
throws ServletException, IOException {
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+mj1Gf792f6E/Gqtpk9DQyxL+kCoMdbgPu/M0/szaGzsJEIwWmZuXVwA+QFBr0UUUBSd0pS9tZxy
9aJu4Ngj5rjzpyjGedBSYFMMkQHqvnPnXjTVKafbK3tCJQfHHGmaKAqOupooCiiigKKKKBO/sheS
2pkxuQyiQjtI5U0Fwcjwr1RQFFFFB//Z";
try{
res.setContentType("image/*"); // 设置返回的⽂件类型
OutputStream toClient = OutputStream();
GenerateImage(xmlImg,toClient);
}catch(Exception ex){
System.out.String());
}
}aaaaaaaaaaaaaaaaaa
// 对字节数组字符串进⾏Base64解码并⽣成图⽚
public static boolean GenerateImage(String imgStr,OutputStream out) {
if (imgStr == null) // 图像数据为空
return false;
BASE64Decoder decoder = new BASE64Decoder();
try {
// Base64解码
byte[] b = decoder.decodeBuffer(imgStr);
for (int i = 0; i < b.length; ++i) {
if (b[i] < 0) {// 调整异常数据
b[i] += 256;
}
}
// OutputStream out = new FileOutputStream(imgFilePath);
out.write(b);
out.flush();
out.close();
return true;
} catch (Exception e) {
return false;
}
}
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